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As well as the shoot-out, the net is closing in on DCI Roz Huntley, who has framed her husband Nick for the murder of Tim Ifield, which she carried out, and things take another turn when she receives a crucial tip-off.

Will DS Arnott ever walk again? (Image: BBC/Bernard Walsh)

Martin said: “Line of Duty is so great because the audience have no idea where the story will go next.

“Jed likes to keep everyone in ­suspense and he’s never predictable. I can promise fans of the show that tonight’s finale won’t disappoint.”

Viewers were left shocked earlier in the series when Martin’s ­character was left in a coma after being beaten by Balaclava Man as he went to visit DCI Huntley’s husband during the hunt for Tim’s killer.

He was seen waking from his coma in the last episode, but such is the secrecy surrounding plotlines, even Martin doesn’t know what lies in store for DS Arnott.

He is simply proud to be part of a show that has become one of the Beeb’s flagships, graduating to a prime slot on BBC One from BBC Two after pulling in 4.5million ­viewers across its first three series.

A fifth series has already been commissioned and Martin admitted it takes a lot of hard work to achieve an authentic London accent.

He films intense, wordy sequences shot in one take for 20 minutes, full of back and forth, sudden revelations, turned tables and police jargon.

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Martin said: “It’s physically and mentally challenging. I’m speaking in an unnatural way for long periods and it exhausts my vocal chords.

“After a while it starts to wear on you a bit, but I like to try and get it spot-on.

“That helps me immerse myself in the character and storylines.”

Sitting on a sofa in the house where he grew up during a visit home, the 32-year-old actor laughed when asked if he thought a lad from Greenock would hit the big time playing a hard-nosed cop. Martin said: “The part of Peter Manuel was probably closer to what I imagined.”